In gas sensors for measuring a specific gas or gas analyzers for determining various gases, infrared radiation is radiated into a defined gas volume and the absorption spectrum is recorded. Each gas molecule possesses a characteristic absorption behavior, so that from the absorption spectrum the composition of the gas can be determined. Since infrared spectroscopy is a non-invasive method, it presents a versatilely applicable method of chemical analysis. Fields of use include, for example, pharmaceutical analysis, quality control in industrial processes, environmental chemistry, as well as the fields of geology and astronomy.
Serving, as a rule, as an infrared radiation source is a thermal radiator, whose surface is heated to sufficiently high temperatures to provide adequate radiation intensity in the desired wavelength range.
A very widely used thermal radiator was the Nernst glower. Furthermore, the so called Globar, a rod of silicon carbide, which is provided with electrodes on its ends, is known. Via the electrodes, electrical current is led through the rod, whereby such is heated and emits radiation principally in the infrared range.
Described in patent EP 0106431 B1 is an infrared radiation source essentially composed of a ceramic rod with a heating wire wound around it, wherein there is provided in the ceramic rod a black body cavity, which ultimately emits the infrared radiation.
A disadvantage of the radiation sources mentioned is that a modulation of the radiation required for gas spectroscopy cannot be attained via modulation of the heating output, because the thermal mass of the radiator is too great. Instead, mechanical elements, such as, for example, aperture plates, are used. This requires extra space, so that a corresponding gas sensor is relatively large.
An alternative to constantly heated radiation sources are emitters based on thin film technology. The emitters are embodied as a conductive layer on a substrate. Via a pulsating heating current sent through the conductive layer, a pulsation of the emitted radiation can be achieved. Disadvantageous in these radiation sources is the high heat loss, which occurs through the direct contact on the substrate.
An example of a radiation source having a layered structure is shown in the Japanese published patent application JP 03025880 A. Disclosed is an infrared heater for heating a semiconductor wafer, which is constructed of a planar base, an insulating layer applied thereon and a heating layer applied on the insulating layer. Advantageous materials for the infrared radiation emitting insulating layer are ceramics such as aluminum oxide, zirconium oxide, silicon carbide and diamond.
Another group of radiation sources comprise helically wound filaments of a suitable metal or a metal alloy, for example, platinum, tungsten, or nickel chromium. An example for this group is to be found in U.S. Pat. No. 7,122,815 B2. These can, indeed, be operated using a pulsating heating current, however, they nevertheless possess a high thermal mass, which leads to high power consumption and, moreover, requires cooling. The thermal mass can, indeed, be decreased by making the helically wound filaments correspondingly thin; this, however, brings the disadvantage that the helically wound filament has a low mechanical durability. Furthermore, for example, a tungsten wire can be operated only in an oxygen free atmosphere, which requires its placement in a housing filled with a protective gas. A housing, for example of glass, however, lessens the intensity of the radiation.